I'm Russian, mu English too not well...
Well, you need IMG TOOL. Download it right now. Then you ned TXD WORKSHOP.
Open by txd workshop .txd file of your car (taxi.txd, or peren.txd...) Export bitamp and draw in "Paint" any picture. Import bitamp back and save .txd. Now take IMG TOOL and import car files (.txd & .dff) into GTA3.img archive. You should replace old files, and make reserve copy. Well, start the game!